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A game of quantum hot potato

Photonics SpectraMay 2011
BOULDER, Colo. – To simplify information processing for quantum
computers and simulations, scientists have coaxed two atoms in separate locations
to take turns jiggling back and forth while swapping the smallest measurable units
of energy.

For the first time, physicists at NIST (National Institute of
Standards and Technology) have enticed two beryllium ions to take turns vibrating
in an electromagnetic trap, exchanging the quanta, or units of energy, that are
a hallmark of quantum mechanics. Their experiment yielded as little as one quantum
of energy traded between the ions, signifying that the charged particles are linked
together. Acting as harmonic oscillators, the ions are similar to a pendulum or
tuning fork, which makes a repetitive back-and-forth motion.

Scientists at NIST used this apparatus to coax two beryllium ions into swapping the smallest
measurable units of energy back and forth, a technique that could simplify information
processing in a quantum computer. Courtesy of Y. Colombe, NIST.
The scientists conducted their experiments using a one-layer ion
trap cooled to –269 °C with a liquid helium bath. They found that the position
of the ions – only 40 µm apart – enabled stronger coupling, while the
cryogenic temperatures prevented distortion of ion behavior.

To begin the energy swapping demonstration, the researchers cooled
both ions with a laser to slow their motion, then cooled them further with two opposing
ultraviolet laser beams to a motionless state. Next, they tuned the voltage of the
trap electrodes to turn on the coupling interaction.

Ion-swapping energy levels were measured every 155 µs at the several-quanta
level and every 218 µs at the single-quantum level. The investigators observed two
round-trip exchanges at the single-quantum level; they also found that the ions
would swap energy indefinitely unless heating disrupted the process.

A similar experiment in 2009 demonstrated entanglement. This time,
however, the scientists coupled the oscillators’ motions more directly than
before. They also observed quantum behavior but, in contrast to the earlier experiment,
did not verify entanglement. The findings appeared online Feb. 23, 2011, in Nature
(doi: 10.1038/nature09721).